The best known methods of fabricating metal parts by UV photolithography and galvanic deposition are those based on LIGA-UV technology. Originally, LIGA (Lithographie Galvanik Abformung) technology was developed by W. Ehrfled of Karlsruhe Kernforschungszentrum (Germany) in the 1980's. It has proved very advantageous for fabricating high precision metal microstructures.
The principle of the LIGA technique consists in depositing a layer of photosensitive resin on a conductive substrate or a substrate coated with a conductive coating, carrying out X radiation via a synchrotron, through a mask that conforms to the contour of the desired microstructure; developing, i.e. removing by physical or chemical means the non-radiated portions of the photosensitive resin so as to define a mould that has the contour of the microstructure, electro-galvanically depositing a metal, typically nickel, in the photosensitive resin mould, then removing the mould to release the microstructure.
The quality of the microstructures obtained is beyond reproach, but the need to implement expensive equipment (the synchrotron) makes this technique incompatible with mass production of microstructures that must have a low unitary cost.
This is why similar methods have been developed on the basis of the LIGA method, but which use ultraviolet irradiation (UV) photosensitive resins. A method of this type is disclosed for example in the publication by A. B. Frazier et al, entitled “Metal Microstructures Fabricated Using Photosensitive Polyimide Electroplating Molds”, Journal of Microelectromechanical Systems, Vol. 2, N deg. 2 June 1003. This article discloses a method of fabricating metal structures by electroplating metal in polyimide based photosensitive resin moulds. The method includes the following steps:                creating, on a substrate, a sacrificial metal layer and a strike layer for a subsequent galvanic deposition step,        applying a layer of photosensitive polyimide,        UV irradiating the polyimide layer through a mask that conforms to the contour of the desired microstructure,        developing by dissolving the non-irradiated parts of the polyimide layer so as to obtain a polyimide mould,        electro-galvanically depositing nickel in the open part of the mould to the top of said mould,        removing the sacrificial layer and separating the metal structure obtained from the substrate, and        removing the polyimide mould.        
The microstructures obtained in accordance with the methods of the prior art are metal microstructures made of a single metal, which is not always optimal, particularly for timepiece making applications. Indeed, it may be advantageous, for aesthetic, tribological or more generally mechanical reasons, to make bimetal microstructures comprised of at least one part made of a first metal, inserted into a part made of a second metal.
Usually, to make such bimetal microstructures, the insert (or inserts) made of a first metal is (or are) added in a traditional manner to the part made of a second metal by setting, crimping, screwing or stamping operations.
EP Patent No. 1,916,567 discloses a mixed method of fabricating parts by photolithography, adding inserts and electroforming. This method requires assembling at least two elements, one obtained by photolithography and galvanic growth, and the other by another fabrication method, and it includes the following steps:                irradiating a photosensitive resin layer applied to a substrate through a mask;        developing the photosensitive layer to form a polymerised resin mould;        placing an added element obtained by another fabrication method in the polymerised resin mould;        galvanic deposition of a metal layer from the bottom of the resin mould, such that the metal layer holds all or part of the added element;        obtaining the part, by separating the substrate from the metal layer holding the added element and removing the polymerised resin mould.        
It will be clear that at the end of the above method, the added element has already been inserted into the fabricated part. Thus, the subsequent setting, crimping, screwing or stamping operations can be omitted.
The method that has just been described has some drawbacks. Since the insert is formed by an element added from elsewhere, it has to be placed in the resin mould with a high level of precision. According to the aforementioned prior document, precise positioning of the added element is obtained via a particular configuration of the polymerised resin mould. Indeed, according to that document, the resin layer is configured such that some portions thereof can act as a guide for the added element. It will be clear that this method considerably limits the choice of shape of the microstructure being made.
It is thus an object of the present invention to provide a method for fabricating bimetal parts that include at least a first element made of a first metal, inserted into a second element made of a second metal. The method allows micrometric precision in positioning the insert, while allowing the greatest possible freedom of choice as to the shape of the microstructure.